const (
	LevelTrace  = slog.Level(-8)
	LevelDbg3   = slog.Level(-7)
	LevelDbg2   = slog.Level(-6)
	LevelDbg1   = slog.Level(-5)
	LevelNotice = slog.Level(2)
	LevelFatal  = slog.Level(12)
)

var LevelNames = map[slog.Leveler]string{
	LevelTrace:  "TRC",
	LevelDbg3:   "D-3",
	LevelDbg2:   "D-2",
	LevelDbg1:   "D-1",
	LevelNotice: "NTC",
	LevelFatal:  "FTL",
}

. . .

		opts := &slog.HandlerOptions{
			Level: LevelTrace,
			ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
				if a.Key == slog.LevelKey {
					level := a.Value.Any().(slog.Level)
					levelLabel, exists := LevelNames[level]
					if !exists {
						levelLabel = level.String()
					}

					a.Value = slog.StringValue(levelLabel)
				}

				return a
			},
		}

		logger := slog.New(slog.NewTextHandler(os.Stdout, opts))

		ctx := context.Background()
		logger.Log(ctx, LevelDbg1, "Hidden debug message level1")
		logger.Log(ctx, LevelDbg2, "Hidden debug message level2")
		logger.Log(ctx, LevelDbg3, "Hidden debug message level3")
		logger.Log(ctx, LevelTrace, "Trace message")
		logger.Log(ctx, LevelNotice, "Notice message")
		logger.Log(ctx, LevelFatal, "Fatal level")